The present invention relates to a thin film capacitor, and, particularly, to a trench type thin film capacitor.
Multifarious surface-mounted type electronic parts are mounted in an internal circuit of an electronic device, such as a computer or a portable terminal. Recently, it is a prime task to make multifarious electronic parts smaller and thinner (thinner films), regardless of whether they are active devices or passive devices, to meet a demand of higher performances and smaller sizes of electronic devices. Some examples of such thin film electronic parts are a thin film capacitor, a thin film inductor, a thin film LC composite part, a thin film lumped parameter device, a thin film distributed parameter device, and a thin film stacked composite part, which are fabricated by using a thin film forming process.
As one scheme of achieving down-sizing and flattening of a thin film capacitor among such thin film electronic devices while increasing the capacitance of the thin film capacitor per unit volume (volumetric capacity) thereof, a method of forming a trench (stepped base structure) at the base substance and forming a thin film capacitor in that trench has been proposed (see Japanese Patent Application Laid-Open No. H06-325970). Since this method increases the surface area of the electrodes of the thin film capacitor as compared with a case of forming a thin film capacitor on a smooth base substance, the capacitance can be made larger.
In principles such a trench type thin film capacitor can increase the electrode surface area per unit area by increasing the aspect ratio of the trench (AR: depth/width), thereby further increasing the capacitance of the capacitor. To make uniform the capacitance of the thin film capacitor between adjoining trenches or in the base substance, such as a wafer, or a substrate surface (hereinafter, collectively called “in the base substance surface” or “in the substrate surface”), it is demanded to make the electrode surface area or the thin film capacitor uniform. This requires that in the case of a trench type thin film capacitor, the area of a side surface which is expressed by the product of the circumferential length of the trench and the height thereof should be made uniform.
In general, typical methods of forming a trench include a method of forming a trench on a metal base substance to be an electrode using a dicing saw, an electron beam or the like as described in the aforementioned Japanese Patent Application Laid-Open No. H06-325970, a method of forming a resist having a trench pattern on a base substance to be an electrode and forming a trench by dry etching or wet etching with the resist used as a mask (subtractive method), and a method of forming a resist with a desired pattern on a base substance and forming (depositing) an additional layer on the base substance at a region other than the resist (additive method). From the viewpoint of securing work precision and shape stability in micropatterning to suppress a change in capacitance, trench formation using the subtractive method and additive method is preferable.
In the subtractive method to form a trench by etching, however, side etching occurs inevitably. Because there is a limit in suppressing a change in the amount of side etching in the base substance surface, the circumferential length of the trench varies within the base substance surface. This scheme tends to make a side-etching oriented change in circumferential length more noticeable as the aspect ratio of the trench becomes higher.
While the additive method can acquire a trench as defined by the size of openings in a patterned resist, the dimensions of the resist pattern itself may vary due to the influence of the lithography process and the developing process. Therefore, the change in the dimensions of the resist pattern in the base substance surface reflects on the trench, causing a change in the circumferential length of the trench in the base substance surface.
As apparent from the above, both of the subtractive method and the additive method have a difficulty in completely eliminating a change in the circumferential length of the trench in the base substance surface. This raises a problem that the change in circumferential length varies (makes non-uniform) the capacitance of the thin film capacitor in the base substance surface.
Accordingly, the present invention has been made in view of the foregoing situation, and it is an object of the invention to provide a thin film capacitor which can improve the uniformity of the capacitance while keeping a high capacitance.